In 1582, Thursday, October 4 was followed by Friday, October 15. In a way, one can say that there were 10 ‘lost’ days. Why was this so? Who decreed this ‘loss’?

On October 5, 1582, Pope Gregory XIII (1502-85, reigned 1572-85) issued his papal bull Inter gravissimas (In the gravest concern) which modified and replaced the Julian calendar then in general use. But why was it felt necessary to effect the change? After all, the Julian and this new Gregorian calendars are both solar calendars.

When the Julian calendar was brought into effect in 45BC by Julius Caesar (hence its name), it was calculated, incorrectly as it turned out, that the average solar year was exactly 365.25 days long, thus necessitating a leap year every four years without exception. The Gregorian calendar corrected this error by shortening the average calendar year to 365.2425 days long, thus shortening the average year by 0.0075 days to stop the drift of the year with respect to the equinoxes. The Julian calculation meant there was an over-estimate of a little under one day every century.

By the year AD325, when the first Council of Nicaea was convened, the excess leap years in the prevailing Julian calendar brought about the fact that the March equinox was occurring well before March 21, its nominal date. This date was important for Christian Churches because the date for the celebration of Easter is calculated on it.

The Julian calculation meant there was an over-estimate of a little under one day every century

Therefore, to reinstate the association between the March equinox and Easter, it was necessary to advance the date by 10 days, and that is why, in 1582, October 4 was followed by October 15. Moreover, a reform was carried out in the lunar cycle used to calculate the Easter date because the astronomical ‘new moons’ were occurring four days before the calculated dates. However, fundamentally, the new calendar continued to be based on the same geocentric theory used for the old calendar.

Like the Julian calendar, the Gregorian calendar consists of 12 months of 28, 30 or 31 days with an extra day added to February in each leap year. The only difference is that the Gregorian reform omitted a leap year in three centennial years every 400 years. Thus, centennial years are leap years only if they are exactly divisible by 400.

To give an example, the years 1700, 1800 and 1900 were not leap years (they were not exactly divisible by 400) but the year 2000 was a leap year. Calendar cycles repeat completely every 400 years, equivalent to 146,097 days. In each cycle, 303 years are regular days of 365 days and 97 years are leap years of 366 days.

The main architect of the Gregorian calendar was the German mathematician Christopher Clavius (1538-1612) while the proposal to reduce the number of leap years in four centuries from 100 to 97 (as outlined above) was by the Calabrian doctor Luigi Lilius Ghiraldi (1510-76) who also produced an original and practical scheme to adjust the epacts of the moon when the annual date for Easter was to be calculated. The epact is the excess of the solar month above the lunar synodical month and of the solar year above the lunar year of 12 synodical months.

Christopher Clavius (1538-1612) who was the main brain behind the making of the Gregorian Calendar. Photo: Wikipedia.com

Luigi Lilio Ghiraldi (1510-76), who was one of the main architects of the Gregorian Calendar, especially with regard to the solving of the ‘Leap Years’ problem. Photo: wikipedia.com

The errors in the Julian calendar had long been pointed out. In the 8th century, the Venerable Bede (died AD725) showed that the accumulated error in his time was over three days; Roger Bacon (1220-92) estimated the error at seven or eight days in c.1200; while Dante Alighieri (1265-1321) in c.1300 was aware of the need for the reform of the calendar.

An attempt to initiate the reform was made by Pope Sixtus IV (1471-84) and, in 1545, the Council of Trent authorised Pope Paul III (1534-49) to reform the calendar. But the project was only implemented under Pope Gregory XIII.

One of the first printed editions of the Gregorian Calendar, in Rome by Vincenzo Accolti in 1582. Photo: Wikipedia.com

A papal brief, dated April 3, 1582, granted a certain Antoni Lilio the exclusive right to publish the new calendar for a period of 10 years, which resulted in the printing of the Lunario Novo secondo la nuova riforma by Vincenzo Accolti at Rome.

The bull Inter gravissimas became the law of the Catholic Church in 1582 but it was not recognised by Protestant, Eastern Orthodox, Oriental Orthodox and a few other Churches. On September 29, 1582, Spain and Portugal and their colonies, together with the papal states and the Polish-Lithuanian Commonwealth, changed over to the new calendar, while France followed suit on December 20, with December 9 being followed by December 20.

Malta, ruled by the Catholic Order of St John, adopted the new calendar immediately. Over the next four centuries, it was adopted, at least for civil use, by practically all other countries and it is now the calendar in general use.